It's not that easy on the hardware side. Keep in mind that the way both Google and Microsoft previously entered this market was by buying an established manufacturer (Motorola and Nokia, respectively). But Microsoft squandered Nokia's manufacturing assets and would need to either start from scratch or acquire somebody else. But there aren't many manufacturers left that are decent, non-Chinese, and willing to sell.

There's also the option to pair with a manufacturer and ask them to put Microsoft's OS on their phones, but Google would most likely lean on anybody attempting that and threaten to revoke their access to Android trademark and Google Services. Samsung is the only manufacturer in a position to tell them to suck it but they're locked into a complex struggle with Google and it's anybody's guess if taking Microsoft on board would help or hinder their position.